Systems, method and apparatus for curing conductive paste

ABSTRACT

One embodiment can provide a system for curing conductive paste applied on photovoltaic structures. The system can include a wafer carrier for carrying a plurality of photovoltaic structures and a heater. The wafer carrier can include a surface element that is in direct contact with the photovoltaic structures and is substantially thermally insulating. The heater can be positioned above the wafer carrier. The heater can include a heated radiation surface that does not directly contact the photovoltaic structures.

CROSS-REFERENCE TO OTHER APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/163,543, entitled “SYSTEMS, METHOD AND APPARATUS FOR CURINGCONDUCTIVE PASTE,” by inventors Edward Sung and James Zu-Yi Liu, filed24 May 2016.

FIELD OF THE INVENTION

This generally relates to the fabrication of photovoltaic structures.More specifically, this disclosure is related to a system and method forcuring conductive paste that is used for electrical and mechanicalcoupling between cascaded strips.

DEFINITIONS

“Solar cell” or “cell” is a photovoltaic structure capable of convertinglight into electricity. A cell may have any size and any shape, and maybe created from a variety of materials. For example, a solar cell may bea photovoltaic structure fabricated on a silicon wafer or one or morethin films on a substrate material (e.g., glass, plastic, or any othermaterial capable of supporting the photovoltaic structure), or acombination thereof.

A “solar cell strip,” “photovoltaic strip,” or “strip” is a portion orsegment of a photovoltaic structure, such as a solar cell. A solar cellmay be divided into a number of strips. A strip may have any shape andany size. The width and length of strips may be the same or differentfrom each other. Strips may be formed by further dividing a previouslydivided strip.

A “cascade” is a physical arrangement of solar cells or strips that areelectrically coupled via electrodes on or near their edges. There aremany ways to physically connect adjacent photovoltaic structures. Oneway is to physically overlap them at or near the edges (e.g., one edgeon the positive side and another edge on the negative side) of adjacentstructures. This overlapping process is sometimes referred to as“shingling.” Two or more cascading photovoltaic structures or strips canbe referred to as a “cascaded string,” or more simply as a “string.”

“Finger lines,” “finger electrodes,” and “fingers” refer to elongated,electrically conductive (e.g., metallic) electrodes of a photovoltaicstructure for collecting carriers.

A “busbar,” “bus line,” or “bus electrode” refers to an elongated,electrically conductive (e.g., metallic) electrode of a photovoltaicstructure for aggregating current collected by two or more finger lines.A busbar is usually wider than a finger line, and can be deposited orotherwise positioned anywhere on or within the photovoltaic structure. Asingle photovoltaic structure may have one or more busbars.

A “photovoltaic structure” can refer to a solar cell, a segment, orsolar cell strip. A photovoltaic structure is not limited to a devicefabricated by a particular method. For example, a photovoltaic structurecan be a crystalline silicon-based solar cell, a thin film solar cell,an amorphous silicon-based solar cell, a poly-crystalline silicon-basedsolar cell, or a strip thereof.

BACKGROUND

Advances in photovoltaic technology, which is used to make solar panels,have helped solar energy gain mass appeal among those wishing to reducetheir carbon footprint and decrease their monthly energy costs. However,the panels are typically fabricated manually, which is a time-consumingand error-prone process. This makes it costly to mass-produce reliablesolar panels.

Solar panels typically include one or more strings of complete solarcells. Adjacent solar cells in a string may overlap one another in acascading arrangement. For example, continuous strings of solar cellsthat form a solar panel are described in U.S. patent application Ser.No. 14/510,008, filed Oct. 8, 2014, and entitled “Module Fabrication ofSolar Cells with Low Resistivity Electrodes,” the disclosure of which isincorporated herein by reference in its entirety. Producing solar panelswith a cascaded cell arrangement can reduce the resistance due tointerconnections between the strips, and can increase the number ofsolar cells that can fit into a solar panel.

Fabrications of such cascaded panels can involve overlapping edges ofadjacent cells in such a way that the electrodes (busbars) on oppositesides of the overlapped cells are in contact to establish an electricalconnection. This process is repeated for a number of successive cellsuntil one string of cascaded cells is created. A number of strings arethen coupled to each other (either in series or in parallel) and placedin a protective frame. To further reduce internal resistance of theentire panel and to ensure that the manufactured panel is compatiblewith conventional panels, one form of the cascaded panel (as describedin the aforementioned patent application) can include a series of solarcell strips created by dividing complete solar cells into smaller pieces(i.e., the strips). These smaller strips can then be cascaded(edge-overlapped) to form a string. Conductive paste can be applied onthe busbars to provide mechanical bonding and electrical couplingbetween the overlapping busbars of adjacent strips.

SUMMARY

A system for curing conductive paste applied on photovoltaic structurescan be provided. The system can include a wafer carrier and a heater.The wafer carrier can carry a plurality of photovoltaic structures andcan include a surface element that is in direct contact with thephotovoltaic structures. The surface element can be substantiallythermally insulating. The heater can be positioned above the wafercarrier, and can include a heated radiation surface that does notdirectly contact the photovoltaic structures.

In some embodiments, the surface element can be made ofpolybenzimidazole (PBI) plastic.

In further embodiments, a surface of the surface element can bepatterned such that only a fraction of the surface is in contact withthe photovoltaic structures.

In some embodiments, the surface element can include a number ofcomponents separated by air gaps to allow an individual component toexpand when heated.

In some embodiments, the temperature of the heated radiation surface canbe kept between 200 and 600° C.

In some embodiments, the heater can include a radiation block, and theradiation surface of the radiation block can be coated with asubstantially dark colored coating.

In further embodiments, the substantially dark colored coating caninclude an anodizing coating or a high-emissivity coating, and thethickness of the dark colored coating can be between 1 and 100 microns.

In further embodiments, other surfaces of the radiation block arepolished or covered with a layer of thermal insulation material.

In some embodiments, the radiation block can be made of a materialhaving thermal conductivity of at least 50 W/(m⋅k).

In some embodiments, the wafer carrier can further include a base forcoupling the wafer carrier to a conveyor system.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A shows an exemplary grid pattern on the front surface of aphotovoltaic structure.

FIG. 1B shows an exemplary grid pattern on the back surface of aphotovoltaic structure.

FIG. 2A shows a string of cascaded strips.

FIG. 2B shows a side view of the string of cascaded strips.

FIGS. 3A and 3B show the busbars and conductive paste before and after,respectively, the conductive paste is cured, according to an embodimentof the present invention.

FIGS. 4A and 4B show the busbars and conductive paste before and after,respectively, the conductive paste is cured, according to an embodimentof the present invention.

FIG. 5 shows an exemplary thermal curing system, according to anembodiment of the present invention.

FIG. 6 shows the perspective view of the heater, according to anembodiment of the present invention.

FIG. 7A shows the top view of an exemplary wafer carrier, according toan embodiment of the present invention.

FIG. 7B shows the cross-sectional view of a strip carrier along cutplane A-A, according to an embodiment of the present invention.

FIG. 7C shows an exemplary placement of two adjacent strips on the stripcarriers, according to an embodiment of the present invention.

FIG. 8A shows the top view of an exemplary wafer carrier, according toan embodiment of the present invention.

FIG. 8B shows the top view of an exemplary wafer carrier, according toan embodiment of the present invention.

FIG. 9A shows an exemplary inline thermal curing system, according to anembodiment of the present invention.

FIG. 9B shows an exemplary inline thermal curing system, according to anembodiment of the present invention.

FIG. 9C shows the end of a wafer carrier moving out of the heated regionbelow the heater, according to an embodiment of the present invention.

FIG. 9D shows the movement of the empty wafer carrier, according to anembodiment of the present invention.

FIG. 9E shows a top view of the wafer carrier, according to anembodiment of the present invention.

FIG. 9F shows an exemplary inline thermal curing system, according to anembodiment of the present invention.

FIG. 10 shows an exemplary process for curing conductive paste appliedonto photovoltaic structures, in accordance with an embodiment of thepresent invention.

FIG. 11 shows an exemplary process for forming a solar panel, accordingto an embodiment.

In the figures, like reference numerals refer to the same figureelements.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the embodiments, and is provided in the contextof a particular application and its requirements. Various modificationsto the disclosed embodiments will be readily apparent to those skilledin the art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present disclosure. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

Overview

Embodiments of the present invention can provide a system and apparatusfor thermally curing conductive paste used for edge-overlapped bondingsolar cell strips. More specifically, the system includes a heater and awafer carrier. The heater can include a metallic block, which canradiate heat to the surface of solar strips, causing the conductivepaste sandwiched between overlapping busbars to be cured. The radiationsurface of the metallic block can be coated, anodized, or roughened tohave a substantially dark color, thus increasing the radiationefficiency. The wafer carrier can be made of materials that areresistant to high temperatures and have a low thermal conductivecoefficient. In some embodiments, at least a portion of the wafercarrier can be made of polybenzimidazole (PBI) plastic. The very lowthermal conductivity of the PBI plastic can ensure that the heat emittedfrom the heater is mostly trapped between the heater and the substratecarrier and, thus, can efficiently cure the conductive paste.

Solar Panel Based on Cascaded Strips

As described in U.S. patent application Ser. No. 14/563,867, a solarpanel can have multiple (e.g., three) strings, each string includingcascaded strips, connected in parallel. Such a multiple-parallel-stringpanel configuration provides the same output voltage with a reducedinternal resistance. In general, a cell can be divided into n₁ strips,and a panel can contain n₂ strings. The numbers n₁ and n₂ can be thesame or different. The number of strips in a string can be a few morethan, a few less than, or the same as the number of regular photovoltaicstructures in a conventional single-string panel. Such a configurationcan ensure that each string outputs approximately the same voltage as aconventional panel. The n₂ strings can then be connected in parallel toform a panel. As a result, the panel's voltage output can be the same asthat of the conventional single-string panel, while the panel's totalinternal resistance can be 1/n₂ of the resistance of a string.Therefore, in general, a greater n₂ can lead to a lower total internalresistance and, hence, more power extracted from the panel. However, atradeoff is that as n₂ increases, the number of connections required tointerconnect the strings also increases, which increases the amount ofcontact resistance. Also, the greater n₁ is, the more strips a singlecell needs to be divided into, which increases the associated productioncost and decreases overall reliability due to the larger number ofstrips used in a single panel.

Another consideration in determining n₁ is the contact resistancebetween the electrode and the photovoltaic structure on which theelectrode is formed. The type of electrode can dictate the number ofstrips. For example, conventional silver-paste or aluminum-basedelectrodes typically cannot produce an ideal resistance between theelectrode and underlying photovoltaic structure. As a result, suchelectrodes may require n₁ to be smaller, rather than larger. This isbecause, as n₁ increases, the number of inter-strip connections alsoincreases, which leads to increased internal series resistance of eachstring, and therefore an overall increased internal resistance of theentire panel. In addition, the greater n₁ is, the more difficult thefabrication process is. In some embodiments of the present invention,the electrodes, including both the busbars and finger lines, can befabricated using a combination of physical vapor deposition (PVD) andelectroplating of copper as an electrode material. The resulting copperelectrode can exhibit lower resistance than an aluminum orscreen-printed-silver-paste electrode. Consequently, a smaller n₁ can beused to attain the benefit of reduced contact resistance per stripwithout incurring too much resistance caused by inter-strip connections.In some embodiments, n₁ can be selected to be three, which is less thanthe n₁ value generally needed for cells with silver-paste electrodes orother types of electrodes. Correspondingly, two grooves can be scribedon a single cell to allow the cell to be divided into three strips.

In addition to lower contact resistance, electroplated copper electrodescan also offer better tolerance to microcracks, which may occur during acleaving process. Such microcracks might adversely impactsilver-paste-electrode cells. Plated-copper electrodes, on the otherhand, can preserve the conductivity across the cell surface even ifthere are microcracks in the photovoltaic structure. The copperelectrode's higher tolerance for microcracks can allow one to usethinner silicon wafers to manufacture cells. As a result, the grooves tobe scribed on a cell can be shallower than the grooves scribed on athicker wafer, which in turn can help increase the throughput of thescribing process. More details on using copper plating to formlow-resistance electrodes on a photovoltaic structure are provided inU.S. patent application Ser. No. 13/220,532, entitled “SOLAR CELL WITHELECTROPLATED GRID,” filed Aug. 29, 2011, the disclosure of which isincorporated herein by reference in its entirety.

FIG. 1A shows an exemplary grid pattern on the front surface of aphotovoltaic structure. In the example shown in FIG. 1A, grid 102 caninclude three sub-grids, such as sub-grid 104. This three sub-gridconfiguration can allow the photovoltaic structure to be divided intothree strips. To enable cascading, each sub-grid can have an edgebusbar, which can be located either at or near the edge. In the exampleshown in FIG. 1A, each sub-grid can include an edge busbar (“edge” hererefers to the edge of a respective strip) running along the longer edgeof the corresponding strip and a plurality of parallel finger linesrunning in a direction parallel to the shorter edge of the strip. Forexample, sub-grid 104 can include edge busbar 106, and a plurality offinger lines, such as finger lines 108 and 110. To facilitate thesubsequent laser-assisted scribe-and-cleave process, a predefined blankspace (i.e., space not covered by electrodes) can be inserted betweenthe adjacent sub-grids. For example, blank space 112 can be defined toseparate sub-grid 104 from its adjacent sub-grid. In some embodiments,the width of the blank space, such as blank space 112, can be between0.1 mm and 5 mm, preferably between 0.5 mm and 2 mm. There is a tradeoffbetween a wider space that leads to a more tolerant scribing operationand a narrower space that leads to more effective current collection. Ina further embodiment, the width of such a blank space can beapproximately 1 mm.

FIG. 1B shows an exemplary grid pattern on the back surface of aphotovoltaic structure. When showing the back surface, for illustrationpurposes, the photovoltaic structure is assumed to be transparent. Thegrid patterns on the front and back surfaces of the photovoltaicstructure are viewed from the same viewing point. In the example shownin FIG. 1B, back grid 120 can include three sub-grids, such as sub-grid122. To enable cascaded and bifacial operation, the back sub-grid maycorrespond to the front sub-grid. More specifically, the back edgebusbar can be located at the opposite edge of the frontside edge busbar.In the examples shown in FIGS. 1A and 1B, the front and back sub-gridscan have similar patterns except that the front and back edge busbarsare located adjacent to opposite edges of the strip. In addition,locations of the blank spaces in back conductive grid 120 can correspondto locations of the blank spaces in front conductive grid 102, such thatthe grid lines do not interfere with the subsequent scribe-and-cleaveprocess. In practice, the finger line patterns on the front and backsides of the photovoltaic structure may be the same or different.

In the examples shown in FIGS. 1A and 1B, the finger line patterns caninclude continuous, non-broken loops. For example, as shown in FIG. 1A,finger lines 108 and 110 can both include connected loops. This type of“looped” finger line pattern can reduce the likelihood of the fingerlines peeling away from the photovoltaic structure after long use.Optionally, the sections where parallel lines are joined can be widerthan the rest of the finger lines to provide more durability and preventpeeling. Patterns other than the one shown in FIGS. 1A and 1B, such asun-looped straight lines or loops with different shapes, are alsopossible.

To form a cascaded string, cells or strips (e.g., as a result of ascribing-and-cleaving process applied to a regular square cell) can becascaded with their edges overlapped. FIG. 2A shows a string of cascadedstrips. In FIG. 2A, strips 202, 204, and 206 can be stacked in such away that strip 206 can partially overlap adjacent strip 204, which canalso partially overlap (on an opposite edge) strip 202. Such a string ofstrips can form a pattern that is similar to roof shingles. Each stripcan include top and bottom edge busbars located at opposite edges of thetop and bottom surfaces, respectively. Strips 202 and 204 may be coupledto each other via an edge busbar 208 located at the top surface of strip202 and an edge busbar 210 located at the bottom surface of strip 204.To establish electrical coupling, strips 202 and 204 can be placed insuch a way that bottom edge busbar 210 is placed on top of and in directcontact with top edge busbar 208.

FIG. 2B shows a side view of the string of cascaded strips. In theexample shown in FIGS. 2A and 2B, the strips can be part of a 6-inchsquare photovoltaic structure, with each strip having a dimension ofapproximately 2 inches by 6 inches. To reduce shading, the overlappingbetween adjacent strips should be kept as small as possible. In someembodiments, the single busbars (both at the top and the bottomsurfaces) can be placed at the very edge of the strip (as shown in FIGS.2A and 2B). The same cascaded pattern can extend along an entire row ofstrips to form a serially connected string.

Conductive Paste Curing System

When forming a solar panel, adjacent strips may be bonded together viaedge busbars. Such bonding can be important to ensure that theelectrical connections have high electrical conductivity and do not failmechanically when the solar panel is put into service. One option forbonding the metallic busbars can include soldering. For example, thesurface of the edge busbars may be coated with a thin layer of Sn.During a subsequent lamination process, heat and pressure can be appliedto cure sealant material between photovoltaic structures and the frontand back covers of the solar panel. The same heat and pressure can alsosolder together the edge busbars that are in contact, such as edgebusbars 208 and 210. However, the rigid bonding between the solderedcontacts may lead to cracking of the thin strips. Moreover, when inservice solar panels often experience many temperature cycles, and thethermal mismatch between the metal and the semiconductor may createstructural stress that can lead to fracturing.

To reduce the thermal or mechanical stress, it can be preferable to usea bonding mechanism that is sufficiently flexible and can withstand manytemperature cycles. One way to do so is to bond the strips usingflexible adhesive that is electrically conductive. For example, adhesive(or paste) can be applied on the surface of top edge busbar 208 of strip202 (shown in FIG. 2A). When strip 204 is placed to partially overlapwith strip 202, bottom edge busbar 210 can be bonded to top edge busbar208 by the adhesive, which can be cured at an elevated temperature.Different types of conductive adhesive or paste can be used to bond thebusbars. In one embodiment, the conductive paste can include aconductive metallic core surrounded by a resin. When the paste isapplied to a busbar, the metallic core establishes an electricalconnection with the busbar while the resin that surrounds the metalliccore functions as an adhesive. In another embodiment, the conductiveadhesive may be in the form of a resin that includes a number ofsuspended conductive particles, such as Ag or Cu particles. Theconductive particles may be coated with a protective layer. When thepaste is thermally cured, the protective layer can evaporate to enableelectrical conductivity between the conductive particles suspendedinside the resin.

In some embodiments, the overlapping busbars can have the shape of arectangular prism, and the conductive paste can applied to the surfaceof at least one of the busbars. Once the conductive paste is cured underheat, the two overlapping busbars will be bonded to each other. FIGS. 3Aand 3B show the busbars and conductive paste before and after,respectively, the conductive paste is cured, according to an embodimentof the present invention. More specifically, FIG. 3A shows that dropletsof conductive paste (e.g., droplets 302 and 304) can be deposited onbusbar 306, which is part of photovoltaic structure 300. FIG. 3B showsthat photovoltaic structures 300 and 310 can be bonded to each otherwhen busbar 312 overlaps with busbar 306 and the conductive pastedroplets are cured.

In some embodiments, the overlapping busbars can have complementaryoverlapping profiles and can interlock when stacked. FIGS. 4A and 4Bshow the busbars and conductive paste before and after, respectively,the conductive paste is cured, according to an embodiment of the presentinvention. FIG. 4A shows that segmented busbars 402 and 412 belonging tophotovoltaic structure 400 and 410, respectively, have complementarytopology profiles. Conductive paste droplets (e.g., droplets 404 and406) can be applied on segments of busbar 402 and exposed surface ofphotovoltaic structure 400. FIG. 4B shows that photovoltaic structures400 and 410 can be bonded to each other when busbars 402 and 412interlock and the conductive paste droplets are cured. Morespecifically, segments of the busbar of one photovoltaic structure canbe bonded via the cured paste to the exposed surface of the otherphotovoltaic structure.

Heat can be used to cure the conductive paste. In conventionalapproaches, cascaded strips (with paste applied and busbars overlapped)can be placed in a convection oven. However, the heating efficiency ofsuch approaches is typically low, because most of the heat may escape tothe environment. For example, when the strips are placed inside aconvection oven, a majority of the heat is used to heat up the air inthe oven. In addition, air must be circulated to ensure that there iseffective and even convection occurring on the strips. An uneven airflowcan result in temperature non-uniformity on the strips. High airflowscan cause the strips to shift position, resulting in incorrect stringgeometry. On the other hand, although low airflows do not shift thestrips, they can lead to low heating efficiency, which not only wastesenergy but can also lead to extended heating time. The longer heatingtime can reduce the throughput of the entire fabrication system.Moreover, long-time exposure of the photovoltaic structures to hightemperatures can also damage the photovoltaic structures.

In another conventional approach, cascaded strips can be placed one byone, or in groups, on a heating surface. While the strips are beingplaced on the surface, the surface must remain cooler than thetemperature required for curing the paste; otherwise, the paste may cureprematurely, before all of the strips have been placed onto the surface.To circumvent this, one may keep the surface relatively cool, and onlyheat it up after all of the strips have been placed. Because theseheating surfaces usually have large thermal masses, heating up andcooling down can take a long time, thus reducing the throughput of thefabrication system. In addition, heating and cooling of a large thermalmass can consume a large amount of energy, and can cause large thermalstresses to be applied to the tool.

To reduce the time needed for curing the conductive paste and to reduceenergy usage, a system that can effectively localize the heat to curethe conductive paste can be provided. FIG. 5 shows an exemplary thermalcuring system, according to an embodiment of the present invention.Thermal curing system 500 can include a wafer carrier 502 for carryingsolar cell strips (e.g., strips 504 and 506) and a heater 508.

In some embodiments, heater 508 can be placed near the surface of thestrips with its radiation surface facing the strips. More specifically,heater 508 does not have direct physical contact with the solar cellstrips. The radiation surface of heater 508 can emit heat (as indicatedby the zigzagged arrows), which can be transferred to and absorbed bythe strips and, hence, indirectly heat up the conductive pastesandwiched between the edge-overlapped strips. After being heated for acertain time period, the conductive paste can be cured, mechanicallybonding the edge-overlapped strips to each other. All three modes ofheat transfer, including conduction, convection, and radiation, can playa role in transferring the heat from heater 508 to the strips, and thento the conductive paste. Among them, radiation plays the most importantrole, i.e., a majority of the heat can be transferred via radiation.Convection is limited to natural convection, and conduction is limitedto the conduction through the air gap between heater 508 and the surfaceof the strips. Compared with direct-contact-based heating, this heatingarrangement can provide higher throughput, improve efficiency, andprevent damage to the strips. Compared with convection-oven-basedheating, this heating arrangement can provide better heating uniformityand higher throughput, and can significantly improve efficiency.

FIG. 6 shows the perspective view of the heater, according to anembodiment of the present invention. Heater 600 can include radiationblock 602 and a number of cartridge heaters (not shown in the drawing).Radiation block 602 can be made of a material with high thermalconductivity, including metallic materials (e.g., aluminum and copper)and ceramic materials (e.g., silicon carbide). In some embodiments, thethermal conductivity of the material forming radiation block 602 can beequal to or greater than 50 W/(m⋅k). Materials with relatively lowthermal conductivity, such as stainless steel and alumina, may also beused, as long as the thermal design of the heater can compensate for thelow thermal conductivity. In the example shown in FIG. 6, radiationblock 602 is shaped as a rectangular prism. Other shapes can also bepossible, such as cubical or cylindrical. Radiation block 602 caninclude a number of voids, extending from one side of radiation block602 to the other. In some embodiments, cartridge heaters, which are atype of heater and can be shaped like rods, can be inserted into thevoids of radiation block 602 to heat radiation block 602 to a hightemperature. These cartridge heaters can be coupled to an external powersource. The amount of heat provided by these cartridge heaters can becontrolled via one or more temperature sensors (not shown in FIG. 6).For example, the system can be configured to maintain the temperature ofradiation block at a predetermined value. In some embodiments, thetemperature of radiation block 602 can be kept at a value between 200and 600° C., preferably between 250 and 350° C.

In the view shown in FIG. 6, front surface 604 of radiation block 602 isthe surface facing the strips and can radiate heat to the strips.Although the entire body of radiation block 602 is heated and allsurfaces of radiation block 602 can radiate heat to the environment,only heat radiated out of surface 604 can be useful for curing theconductive paste on the photovoltaic structure. To increase the amountof heat radiated out of surface 604, in some embodiments, surface 604can be anodized to have a darker (almost black) color to increase itsemissivity. On the other hand, to reduce the amount of heat radiatedfrom other surfaces, all other surfaces are polished to be mirror-liketo reduce their emissivity. Alternatively, all other surfaces can becovered or coated with a layer of thermal insulation material to reducetheir emissivity. In general, radiation block 602 can be configured insuch a way that only one surface has high emissivity and all othersurfaces are effectively insulated.

In addition to anodizing, other methods can also be used to increase theemissivity of surface 604. For example, surface 604 can be roughened.Alternatively, surface 604 can be coated with a thin layer ofradiation-absorbent material (RAM) coating. For example, an aluminumradiation block can have its radiation surface coated with a layer ofTufram® (registered trademark of General Magnaplate Corporation ofLinden, N.J.) coating. The thickness of the coating can be between 1 and100 microns. In alternative embodiments, a high-emissivity coating,which can be a silicone-ceramic based, black pigmented coating, can beused to coat radiation surface 604. The high-emissivity coating can havea thickness between 1 and 100 microns.

In conventional high-temperature settings, metallic or graphite wafercarriers are often used to support wafers due to their heat-resistantcapability. However, although they are not easily damaged by heat, theyare also good heat conductors. After being transferred from theradiation block to the strips located on the wafer carriers, a portionof the heat can escape via the wafer carrier due to its good heatconductivity. This can reduce the heating efficiency and, hence, canprolong the time needed for curing the paste. To overcome this problem,instead of metal or graphite, the wafer carrier can be made of a plasticor ceramic material that is heat-resistant and has low thermalconductivity.

FIG. 7A shows the top view of an exemplary wafer carrier, according toan embodiment of the present invention. Wafer carrier 700 can include abase 702 and a number of strip carriers, such as strip carriers 704,706, and 708. Base 702 is not in contact with the photovoltaicstructures and can be made of materials with good thermal conductivity,such as metal. In one embodiment, base 702 can be made of stainlesssteel. The base of wafer carrier 700 can be used to couple wafer carrier700 to a conveyor system, which can transport the photovoltaicstructures from one processing station to the next.

The strip carriers can be configured to carry the strips that areprecisely aligned such that the busbars of adjacent strips overlap withconductive paste sandwiched in between. A robotic system with theassistance of a vision system can be used to place and align the strips.A detailed description of the robotic system and the vision system canbe found in U.S. patent application Ser. No. 14/866,766, entitled“SYSTEMS AND METHODS FOR CASCADING PHOTOVOLTAIC STRUCTURES,” filed Sep.25, 2015, the disclosure of which is incorporated herein by reference inits entirety.

Because the strip carriers are in direct contact with the heated strips,to reduce heat loss, the strip carriers can be made of a plasticmaterial that is heat-resistant and has low thermal conductivity. Insome embodiments, the strip carriers can be made of polybenzimidazole(PBI) plastic. Compared with other plastic materials, PBI plastic canprovide a number of superior thermal properties, including high thermalresistance, very low heat conductivity, and very low thermal expansioncoefficient. In addition, PBI plastic can also provide superiormechanical properties over other insulating materials, making it anideal candidate for forming the strip carriers.

In addition to choosing a material with low thermal conductivity to formthe strip carriers, in some embodiments, the geometry of the stripcarries is also carefully designed to further reduce the heat loss. Inthe example shown in FIG. 7A, the surface of the strip carriers can bepatterned. More specifically, each rectangle (e.g., rectangle 722) on astrip carrier indicates a raised boss. FIG. 7B shows the cross-sectionalview of a strip carrier along cut plane A-A, according to an embodimentof the present invention. When a strip is placed on the strip carrier,only the ridges (e.g., ridges 732 and 734) are in contact with thestrip. This way, instead of having the entire back surface of the stripsin contact with the strip carriers, only a portion of the back surfaceof the strips is in contact with the strip carrier. In some embodiments,the total size of the top surface of the ridges can be between 10 and30% the size of a strip carrier. The reduced contact area can reduce theamount of heat being transferred from the strips to the strip carriers.In addition to the pattern shown in FIG. 7A, other patterns are alsopossible. For example, instead of rectangular, shapes of theindentations can include square, triangular, circular, half circular,oval, or any other regular or irregular shapes.

In some embodiments, the strip carriers can be designed in such a waythat each strip carrier can support and carry a single strip. Forexample, strip carrier 704 can support one strip, and strip carrier 706can support an adjacent strip. The size of the strip carriers can bedefined based on the size of the strip.

More specifically, each strip carrier can be designed to be smaller thanthe strip to allow a gap to exist between two adjacent strip carriers.For strips that were obtained by dividing standard six-inch squarewafers, the length of the strip can be roughly six inches, whereas thewidth of each strip can be less than two inches (e.g., between 1.5 and1.7 inches).

As shown in FIG. 7A, gap 712 can exist between strip carriers 704 and706, and gap 714 can exist between strip carriers 706 and 708. Insertinga gap between adjacent strip carriers can be important, because thestrip carriers can expand under high temperature. Note that, althoughthe thermal expansion coefficient of PBI plastic is very low, it canstill be higher than that of the stainless steel base. Hence, to preventbowing, it can be desirable to allow the strip carriers to expand. Inthe design shown in FIG. 7A, instead of having a continuous stripcarrier carry multiple strips, separate strip carriers with gaps inbetween are used to carry the multiple strips. The width of the gap canbe between 2 and 10 mm, preferably between 5 and 8 mm.

The way the strips are placed on wafer carrier 700 can also play animportant role in the curing process. FIG. 7C shows an exemplaryplacement of two adjacent strips on the strip carriers, according to anembodiment of the present invention. Strips 742 and 744, shown indifferent hatch patterns, are placed adjacent to each other. Forillustration purposes, in FIG. 7C, strips 742 and 744 are shown astransparent to reveal the strip carriers beneath. To maintain theposition of the strips, each strip carrier can include a number ofvacuum-holding holes, such as holes 746 and 748.

In the example shown in FIG. 7C, the right edge of strip 742 overlapswith the left edge of strip 744. This can result in the overlapping ofthe corresponding busbars (not shown in the drawing) of strips 742 and744. In some embodiments, strips 742 and 744 can be placed in such a waythat their overlapping edges (the crosshatched region shown in FIG. 7C)are positioned above the gap between the two adjacent strip carriers. Inother words, the edges are not supported by the strip carriers directly.Compared with other arrangements, such as having the overlapping edgeson top of a strip carrier, this arrangement can be beneficial to thecuring of the conductive paste. Both strips are pulled down against thestrip carrier by means of vacuum. In the overlapped region, the edgethat is on the bottom will deflect slightly downwards, while the edgethat is on the top will deflect slightly upwards, because they aretrying to occupy the same space. This allows both edges to be locallyparallel to each other, forming the best geometry for bonding. If theoverlapped region were directly supported by the strip carrier, thebottom edge would be flat, while only the top edge would deflectupwards. The two edges would no longer be parallel but form an angle,which is not the ideal bonding geometry. The conductive paste typicallycan cure better when it is heated under pressure.

In addition to the example shown in FIG. 7A, the wafer carrier can haveother forms. FIG. 8A shows the top view of an exemplary wafer carrier,according to an embodiment of the present invention. Wafer carrier 800can include a base 802 and a number of strip carriers, each of which canbe designed to carry a single strip. Unlike the strip carriers shown inFIG. 7A, in FIG. 8A, each strip carrier can include multiple separateplastic pieces. For example, strip carrier 810 can include plasticpieces 812, 814, and 816. Using smaller pieces of plastic to constructthe strip carrier can reduce material cost. More specifically, this canallow different plastic materials to be used to form a strip carrier. Inthe example shown in FIG. 8A, the center piece of a strip carrier can bemade of PBI plastic, whereas the two side pieces can be made of adifferent plastic material, which can have less ideal thermal propertiesbut can cost less than the PBI. Similar to what's shown in FIG. 7A, therectangles in each plastic piece indicate indentations, and the circlesindicate vacuum-holding holes. To match the conductivity of the sidepieces to that of the center piece, one can pattern the side pieces insuch a way that a smaller portion of a side piece is in direct contactwith the back surface of the strip.

FIG. 8B shows the top view of an exemplary wafer carrier, according toan embodiment of the present invention. Wafer carrier 850 can be similarto wafer carrier 800, and can include three strip carriers, with eachstrip carrier including three separate plastic pieces. For example,strip carrier 860 can include side pieces 862 and 866 and center PBIpiece 864. Center piece 864 can include the vacuum-holding holes forholding down the strip. In the example shown in FIG. 8B, each plasticpiece of a strip carrier can be around six inches long and about 0.5inch wide. The gap between the plastic pieces within the same stripcarrier can be about 0.1 inch.

Returning to FIG. 5, in this example, the thermal curing system includesone heater and one wafer carrier, both of which can remain stationaryduring the curing process. The distance between the radiation surface ofthe heater and the strips on the wafer carrier can be kept between 2 and20 mm. Bringing the radiation surface closer to the strips can increasethe heating efficiency and shorten curing time. However, if theradiation surface is too close to the strip surface, the hightemperature may damage the photovoltaic structure. In addition, a veryshort heating time may cause the conductive paste to be heated unevenly.In some embodiments, the temperature of the heater can be maintained ataround 300° C., the distance between the radiation surface and the stripsurface can be about 5 mm, and the wafer carrier can be made of PBIplastic. In such scenarios, the conductive paste can be cured in about60 seconds. In other words, the overlapping strips can be bonded afterbeing placed under the heater for about 60 seconds. If the strips areremoved before the required time for curing, the paste may not besufficiently cured, which can resulting in poor bonding quality. On theother hand, keeping the strips under the heater longer can reduce theoverall system throughput and can potentially damage the strips. In someembodiments, the edge-overlapped strips can be heated for a durationbetween 10 and 100 seconds, depending on the temperature and thermaldesign of the heater, as well as the thermal design of the wafercarrier. A well-designed system that can efficiently heat the stripswithout damaging the photovoltaic junctions can reduce the time neededfor curing the conductive paste to a period between 25 and 60 seconds.

For large scale manufacturing, to increase throughput, an inline thermalcuring system can be used. In an inline curing system, the wafer carrieralong with the strips can be placed on a conveyor system and move undermultiple heaters. FIG. 9A shows an exemplary inline thermal curingsystem, according to an embodiment of the present invention. Inlinethermal curing system 900 can include conveyor system 902, which carrieswafer carrier 904, and a number of heaters, such as heaters 912, 914,916, and 918. A number of photovoltaic strips can be held on the surfaceof wafer carrier 904 by a vacuum-holding mechanism. The surface of wafercarrier 904 can be made of PBI plastic, which is a good heat insulator.The strips have been aligned to each other such that their correspondingedge busbars overlap. Sandwiched between the overlapping busbars is theto-be-cured conductive paste.

During operation, conveyor system 902 can first move wafer carrier 904under heater 912 and then remain stationary to allow wafer carrier 904to stay under heater 912 for a predetermined time. For example, wafercarrier 904 can stay under heater 912 for about 15 seconds.Subsequently, conveyor system 902 can move in a direction indicated byarrow 910 and move wafer carrier to a position under heater 914. Oncewafer carrier 904 is in position, conveyor system 902 can stop again fora predetermined time. The same process can repeat until conveyor system902 moves wafer carrier 904 to a position under last heater 918 andremains there for a predetermined time period. Subsequently, the bondedstrips can be removed from wafer carrier 904 and carried by conveyorsystem 902 to a next processing station. Wafer carrier 904 can bebrought back to starting point of inline thermal curing system 900 tocarry a new set of strips. In some embodiments, conveyor system 902 canbe configured in such a way that wafer carrier 904 can stay under eachheater for an equal amount of time. The total heating time of thestrips, for the example shown in FIG. 9A, can then be four times thetime the strips spent under each individual heater. By carefullyprogramming conveyor system 902, one can ensure that the total heatingtime of the strips can be sufficient to cure the conductive paste. Forexample, if the strips spend about 15 seconds under each heater, thetotal heating time can be roughly 60 seconds, which can be sufficient tocure the conductive paste, given that each heater is kept at about 300°C.

In some embodiments, the heaters (e.g., heaters 912-918) can beconfigured to have different temperatures. More specifically, thetemperature of the front (left side in FIG. 9A) heater (e.g., heater912) can be lower than that of the ones at the back (e.g., heaters 916and 918). The temperature can rise sequentially from the front heatersto the back heaters. For example, heater 914 can have a highertemperature than heater 912, heater 916 can have a higher temperaturethan heater 914, and heater 918 can have a higher temperature thanheater 916. This way, when strips on wafer carrier 904 are moving alongwith conveyor system 902, they can pass through heating zones withincreasing temperatures, thus preventing the strips from sufferingthermal shock.

The implementation of conveyor system 902 can make it possible forparallel curing of multiple groups of strips. More specifically, whenwafer carrier 904 is moved from heater 912 to heater 914, a differentwafer carrier carrying a different group of strips can be positionedunder heater 912, and both groups of strips can be then heatedsimultaneously. This process can repeat with conveyor system 902simultaneously moving multiple groups of strips under the multipleheaters, increasing the system throughput fourfold.

FIG. 9B shows an exemplary inline thermal curing system, according to anembodiment of the present invention. In FIG. 9B, inline thermal curingsystem 920 can include conveyor system 922, which carries wafer carrier924, and extended heater 926. Extended heater 926 can be much larger insize than individual heaters 912-918 shown in FIG. 9A, thus allowingmany more strips to be heated simultaneously. The bottom surface ofheater 926, which can be the radiation surface of heater 926, caninclude a number of divided sections (e.g., section 928). Alternatively,extended heater 926 can include multiple sections (e.g., radiationblocks) placed inside a same physical enclosure. These multiple sectionscan also be configured to have different temperatures to allowphotovoltaic strips to go through heating zones with increasingtemperature when moving along conveyor system 922 in the direction shownby arrow 930.

During fabrication, wafer carrier 924 can start from a location outsideof the area below extended heater 926. In the example shown in FIG. 9B,wafer carrier 924 can start from a location to the left of extendedheater 926. A robotic arm (not shown in FIG. 9B) can pick up a set ofphotovoltaic strips and place the strips on wafer carrier 924. Thestrips can be arranged in such a way that adjacent strips overlap at theedge. In some embodiments, the robotic arm can pick up and arrange threestrips each time. Once a set of strips is placed on wafer carrier 924,conveyor system 922 can move wafer carrier 924 to place the frontportion of wafer carrier 924 under extended heater 926, as shown in FIG.9B. In some embodiments, conveyor system 922 can continue to move untilthe last overlapping edge of the set of strips is under extended heater926, as indicated by dashed line 932. Conveyor system 922 can then pausefor a predetermined time (e.g., between 10 and 15 seconds) to allow theset of strips and, thus, conductive paste sandwiched between theoverlapping edges to be heated by extended heater 926.

While conveyor system 922 remains stationary, the robotic arm can layanother set of strips (e.g., strip set 934) onto wafer carrier 924. Thenew set of strips can be arranged to be edge-overlapped with the set ofstrips that was already on wafer carrier 924 to form a longer string.After conveyor system 922 has remained stationary for the predeterminedtime, conveyor system 922 can move forward again to place the newly laidset of strips under extended heater 926 for heating. This process canrepeat itself until a desired number of strips has been laid onto wafercarrier 924 and has moved through extended heater 926. The total amountof time that a set of strips remains under extended heater 926 (i.e.,the total amount of heating time) can be determined based on thetemperature setting of extended heater 926. For example, if thetemperature of extended heater 926 is set to be at around 300° C., thetotal amount of time used to move a set of strips from one end ofextended heater 926 to the other end of extended heater 926 can beroughly 60 seconds. Considering that conveyor system 922 may pausemultiple times during the duration within which a particular set ofstrips remains under extended heater 926, the time duration for eachpause can be determined based on the total heating time and the numberof pauses. For example, if it takes conveyor system 922 four pauses inorder to move a particular set of strips from one end of extended heater926 to the other end, each pause may last for about 15 seconds. FIG. 9Cshows the end of a wafer carrier moving out of the heated region belowthe heater, according to an embodiment of the present invention.

Once all strips for a string have been moved out of the heated regionunder the extended heater, meaning that the conductive paste has beencured to mechanically bond the strips together, the entire string can beremoved from the wafer carrier. In some embodiments, a robotic arm canpick up the string and transfer it to the next processing station (e.g.,the panel assembly station). Moreover, the extended heater can move upto be further away from the conveyor system in order to reduce theamount of heat radiated onto the conveyor system. This is because theconveyor system is currently not covered by the wafer carrier having aheat-insulating surface. Moving the extended heater further away fromthe conveyor system can also make it easier for loading and unloading ofa wafer carrier. In some embodiments, after the removal of the string,the conveyor system can reverse its direction to move the empty wafercarrier back to the other end of the extended heater to allow new stripsto be loaded onto the wafer carrier and moved into the heated zones. Inalternative embodiments, the conveyor system can continue to move in thesame direction with another wafer carrier being placed onto thebeginning end of the extended heater.

FIG. 9D shows the movement of the empty wafer carrier, according to anembodiment of the present invention. After the photovoltaic string hasbeen removed from wafer carrier 924, conveyor system 922 reverse itsmoving direction (indicated by arrow 936) to move empty wafer carrier924 back to the left side of extended heater 926. In addition, extendedheater 926 can move up to a predetermined location. In some embodiments,the distance between the radiation surface of extended heater 926 andthe top surface of conveyor system 922 can be between 10 and 20 mm. Onthe return trip, wafer carrier 924 does not need to pause.

Wafer carrier 924 can be much larger than wafer carrier 904 and cancarry a relatively large number of photovoltaic strips. In someembodiments, wafer carrier 924 can be configured to carry an entirestring of photovoltaic structures for a solar panel. For example, wafercarrier 924 can hold between 15 and 40 strips. The surface of wafercarrier 924 can be covered by multiple segments of PBI (which can besimilar to PBI segments 814 and 864, shown in FIGS. 8A and 8B,respectively) to ensure that the carried strips are thermally insulatedfrom the base of wafer carrier 924. FIG. 9E shows a top view of thewafer carrier, according to an embodiment of the present invention. Thesurface of wafer carrier 940 can be covered with a number of PBIsegments, such as PBI segments 942 and 946. An air gap can exist betweenadjacent PBI segments to allow individual PBI segments to expand whenheated, thus preventing wafer carrier 940 from surface warping.

In the examples shown in FIGS. 9A-9D, the conveyor system operates in astop-and-go fashion, which can be less ideal because a stop-and-goconveyor may be prone to mechanical failure. To overcome this problem,in some embodiments, the conveyor system can be configured to movecontinuously. FIG. 9F shows an exemplary inline thermal curing system,according to an embodiment of the present invention. Inline thermalcuring system 950 can include conveyor system 952, which carries wafercarrier 954, and single continuous heater 956. Heater 956 can extendalong the length of conveyor system 952. Wafer carrier 954 can includean insulation surface that can be made of PBI plastic segments, similarto wafer carrier 940 shown in FIG. 9E. The insulation surface can be indirect contact with photovoltaic strips carried by wafer carrier 954.

During operation, a number of strips that make an entire string can belaid onto wafer carrier 954 before wafer carrier 954 is sent to theheated region under heater 956. For example, if a string includes 21strips, all 21 strips will be laid in an edge-overlapping fashion ontothe surface of wafer carrier 954. Heater 956 can be maintained at aninitial position that is relatively far away (e.g., between 10 and 20cm) from the surface of conveyor system 952. Subsequent to all thestrips having been loaded onto wafer carrier 954, heater 956 can movedown to be close to the surface of conveyor system 952, and conveyorsystem 952 can start to move wafer carrier 954 to the right to be belowheater 956. In some embodiments, when wafer carrier 954 is at leastpartially below heater 956, the distance between the radiation surfaceof heater 956 and the photovoltaic strips carrier on wafer carrier 954can be between 2 and 10 mm, preferably between 2 and 5 mm. Therelatively small distance ensures good heating efficiency. Becauseheater 956 extends along the direction in which conveyor system 952moves, photovoltaic strips carried by wafer carrier 924 can be heated byheater 956 while moving along with conveyor system 952. The length ofheater 956 and the moving speed of conveyor system 952 can determine thetotal amount of time a strip spends underneath heater 956. As discussedpreviously, the amount of time a strip spends underneath heater 956should be sufficiently long to ensure that conductive paste applied onthe strip can be cured. In some embodiments, it may take a strip about60 seconds to travel from one end of heater 956 to the other end.

In the example shown in FIG. 9A, the size of each heater can becomparable to the size of the wafer carrier. Hence, in FIG. 9A, thethermal curing system may be used to bond, each time, a group of stripsfitted onto a single wafer carrier to form shorter strings. For example,each short string may have three strips. The shorter strings can laterbe bonded to each other to form a longer string. On the other hand, inthe examples shown in FIGS. 9B-9F, the heater and the wafer carrier canbe much longer. More specifically, the wafer carrier can be long enoughto carry a larger number of strips, which can be bonded simultaneouslyto form a longer string. For example, the longer string can have between15 and 40 strips. Being able to form longer strings directly fromindividual strips can increase the system throughput.

FIG. 10 shows an exemplary process for curing conductive paste appliedonto photovoltaic structures, in accordance with an embodiment of thepresent invention. During operation, photovoltaic structures appliedwith conductive paste can be loaded onto a wafer carrier (operation1002). The wafer carrier can include a surface that is in direct contactwith the photovoltaic structures, and such a surface can be made withthermally insulating and heat-resistant materials. In some embodiments,the surface of the wafer carrier can be made of PBI plastic.

Subsequently, the wafer carrier along with the photovoltaic structurescan be brought to the vicinity of a heater (operation 1004). The heatercan include a radiation surface that can efficiently radiate heat in aparticular direction. In some embodiments, the radiation surface can beconfigured to radiate heat in a downward direction, and the wafercarrier can be brought to a location underneath the radiation surface.In further embodiments, the distance between the radiation surface andthe wafer carrier can be between 2 and 10 mm to allow efficient heating.The wafer carrier and the photovoltaic structures can remain in theheated zone under the radiation surface for a predetermined time periodto ensure proper curing of the conductive paste (operation 1006). Thephotovoltaic structure can remain stationary or move along a conveyorsystem while being heated. After the conductive paste is cured, thephotovoltaic structures can be removed from the wafer carrier (operation1008).

FIG. 11 shows an exemplary process for forming a solar panel, accordingto an embodiment. During fabrication, a semiconductor multilayerstructure can be prepared (operation 1102). The semiconductor multilayerstructure can include the base, the emitter, the surface field layer,and one or more transparent conductive oxide (TCO) layers. Thesemiconductor multilayer can also optionally include quantum tunnelingbarrier (QTB) layers on one or both sides of the base layer. Thesemiconductor multilayer structure can then go through a metallizationprocess, which can form a metallic grid on both surfaces of thesemiconductor multilayer structure (operation 1104). Differentmetallization techniques can be used to form the metallic grids. Forexample, an electroplating process that uses a wax-based plating maskcan be used to form the metallic grids.

Subsequently, the photovoltaic structure can optionally be divided intosmaller strips (operation 1106), and conductive paste can be applied onan edge busbar of each strip or photovoltaic structure (operation 1108).A number of strips can be placed onto a specially designed wafer carrierhaving a heat insulation surface, with adjacent strips overlapping atthe edges (operation 1110). As a result, the edge busbars of theadjacent strips overlap and the conductive paste is sandwiched betweenthe overlapping busbars.

The specially designed wafer carrier can then be placed under a heaterthat radiates heat for a predetermine time to cure the conductive paste(operation 1112). Note that the wafer carrier may remain stationary ormay move along a conveyor system during the conductive-paste-curingprocess. In some embodiments, the heater can include a radiation blockhaving a temperature of about 300° C., and the wafer carrier can beplaced under the radiation block for at least 60 seconds. The curing ofthe conductive paste can result in the formation of strings. Finally,the strings can be interconnected to form a panel (operation 1114).

In general, embodiments of the present invention can provide a novelconductive-paste-curing system. The novel system relies on radiation forheat transfer, which can provide better heating uniformity. Theefficiency of the system can be improved by the novel design of theradiation block and a wafer carrier with an insulation surface. Thethroughput of the system can be improved by implementing a conveyorsystem for inline operation.

The foregoing descriptions of various embodiments have been presentedonly for purposes of illustration and description. They are not intendedto be exhaustive or to limit the present invention to the formsdisclosed. Accordingly, many modifications and variations will beapparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention.

What is claimed is:
 1. A system for curing a conductive paste applied totwo adjacent photovoltaic structures, comprising: a wafer carrier forcarrying the two adjacent photovoltaic structures on a first side of thewafer carrier, wherein the two photovoltaic structures are coupled in acascaded manner by the conductive paste; wherein the wafer carrierincludes a surface element that is in direct contact with thephotovoltaic structures and is substantially thermally insulating; and aheater positioned adjacent to the first side of the wafer carrier,wherein the heater includes a heated radiation surface that faces thephotovoltaic structures without directly contacting the photovoltaicstructures, and wherein the heated radiation surface is configured toradiate heat to the two photovoltaic structures for a predeterminedduration to cure the conductive paste.
 2. The system of claim 1, whereinthe surface element comprises polybenzimidazole (PBI) plastic.
 3. Thesystem of claim 1, wherein a surface of the surface element is patternedsuch that a fraction of the surface is in contact with the photovoltaicstructures.
 4. The system of claim 1, wherein the surface elementincludes a number of components separated by air gaps to allow anindividual component to expand when heated.
 5. The system of claim 1,wherein the heated radiation surface has a temperature between 200 and600° C.
 6. The system of claim 1, wherein the heater includes aradiation block, and wherein the heated radiation surface of theradiation block is coated with a substantially dark colored coating. 7.The system of claim 6, wherein the substantially dark colored coatingincludes an anodizing coating or a high-emissivity coating, and whereina thickness of the dark colored coating is between 1 and 100 microns. 8.The system of claim 6, wherein other surfaces of the radiation block arepolished or covered with a layer of thermal insulation material.
 9. Thesystem of claim 6, wherein the radiation block is made of a materialhaving a thermal conductivity of at least 50 W/(m·k).
 10. The system ofclaim 1, further comprising a conveyor system configured to transfer thewafer carrier.
 11. The system of claim 1, wherein the predeterminedduration is between 25 and 60 seconds.
 12. A solar module fabricationmethod, comprising: obtaining a plurality of photovoltaic structures,wherein a photovoltaic structure includes a first edge busbar on a firstedge of a first surface and a second edge busbar on an opposite edge ofan opposite surface; applying conductive paste on the first edge busbarof each photovoltaic structure; aligning the photovoltaic structures ona wafer carrier in such a way that the first edge busbar of a firstphotovoltaic structure overlaps the second edge busbar of an adjacentphotovoltaic structure with the conductive paste sandwiched in between;positioning the wafer carrier to a vicinity of a heated radiationsurface for a predetermined duration such that heat transferred from theheated radiation surface to the photovoltaic structures cures theconductive paste, thereby mechanically bond the photovoltaic structureto form a string.
 13. The solar module fabrication method of claim 12,further comprising: placing the wafer carrier on a conveyer; andcontrolling movement of the conveyor to position the wafer carrier tothe vicinity of the heated radiation surface.
 14. The solar modulefabrication method of claim 12, wherein the wafer carrier includes asurface element that is in direct contact with the photovoltaicstructures and is substantially thermally insulating.
 15. The solarmodule fabrication method of claim 14, wherein the surface element ofthe wafer carrier comprises polybenzimidazole (PBI) plastic.
 16. Thesolar module fabrication method of claim 14, wherein the surface elementincludes a number of components separated by air gaps to allow anindividual component to expand when heated.
 17. The solar modulefabrication method of claim 12, wherein the heated radiation surface ismaintained at a predetermined temperature between 200 and 600° C. 18.The solar module fabrication method of claim 12, wherein the heatedradiation surface of the heating block is coated with a substantiallydark colored coating.
 19. The solar module fabrication method of claim18, wherein the substantially dark colored coating includes an anodizingcoating or a high-emissivity coating, and wherein a thickness of thedark colored coating is between 1 and 100 microns.
 20. The solar modulefabrication method of claim 18, wherein other surfaces of the heatingblock are polished or covered with a layer of thermal insulationmaterial.